Engineers use compression (also called source coding or source encoding) to reduce the bit rate of digital video. Compression decreases the cost of storing and transmitting video information by converting the information into a lower bit rate form. Decompression (also called decoding) reconstructs a version of the original information from the compressed form. A “codec” is an encoder/decoder system.
Over the last two decades, various video codec standards have been adopted, including the ITU-T H.261, H.262 (MPEG-2 or ISO/IEC 13818-2), H.263 and H.264 (MPEG-4 AVC or ISO/IEC 14496-10) standards, the MPEG-1 (ISO/IEC 11172-2) and MPEG-4 Visual (ISO/IEC 14496-2) standards, and the SMPTE 421M standard. More recently, the HEVC standard (ITU-T H.265 or ISO/IEC 23008-2) has been approved. Extensions to the HEVC standard (e.g., for scalable video coding/decoding, for coding/decoding of video with higher fidelity in terms of sample bit depth or chroma sampling rate, or for multi-view coding/decoding) are currently under development. A video codec standard typically defines options for the syntax of an encoded video bitstream, detailing parameters in the bitstream when particular features are used in encoding and decoding. In many cases, a video codec standard also provides details about the decoding operations a decoder should perform to achieve conforming results in decoding. Aside from codec standards, various proprietary codec formats define other options for the syntax of an encoded video bitstream and corresponding decoding operations.
In base color index map (“BCIM”) mode, a video encoder encodes sample values using index values that represent base colors. Each of the index values is associated with a different value (“base color”) among the sample values. For example, if an 8×8 block has four different values among the 64 sample values of the block, the encoder assigns four different index values for those four base colors. During encoding, the sample values are replaced with corresponding index values. The encoder encodes and signals a mapping of index values to base colors (“base color table”) as well as data for the arrangement of index values representing the sample values (“index map”). A decoder receives and decodes the mapping of index values to base colors. Using that mapping, the decoder replaces index values in the index map with base colors for the original sample values.
BCIM mode can be helpful when encoding certain “artificially” created video content such as screen-capture content. Screen-capture content typically includes repeated structures (e.g., graphics, text characters) that use few distinct sample values, compared to natural video. This provides opportunities for BCIM to improve performance.
The current design of BCIM mode for the HEVC standard and implementation in some reference software for the HEVC standard has several problems. For example, BCIM requires many new syntax elements and syntax structures for the base color mapping and index map, which complicates implementation. Also, the approach to encoding/decoding the base color mapping and index map uses tools unlike those already included in the HEVC standard, again complicating implementation. Also, encoding of the base color table and index map is inefficient.